The structure of an active matrix OLED or AM-OLED is well known. It comprises:                an active matrix containing, for each cell, an association of several thin film transistors (TFT) with a capacitor connected to an OLED material; the capacitor acts as a memory component that stores a value during a part of the video frame, this value being representative of a video information to be displayed by the cell during the next video frame or the next part of the video frame; the TFTs act as switches enabling the selection of the cell, the storage of a data in the capacitor and the displaying by the cell of a video information corresponding to the stored data;        a row or gate driver that selects line by line the cells of the matrix in order to refresh their content;        a column or source driver that delivers the data to be stored in each cell of the current selected line; this component receives the video information for each cell; and        a digital processing unit that applies required video and signal processing steps and that delivers the required control signals to the row and column drivers.        
Actually, there are two ways for driving the OLED cells. In a first way, each digital video information sent by the digital processing unit is converted by the column drivers into a current whose amplitude is proportional to the video information. This current is provided to the appropriate cell of the matrix. In a second way, the digital video information sent by the digital processing unit is converted by the column drivers into a voltage whose amplitude is proportional to the video information. This current or voltage is provided to the appropriate cell of the matrix.
From the above, it can be deduced that the row driver has a quite simple function since it only has to apply a selection line by line. It is more or less a shift register. The column driver represents the real active part and can be considered as a high level digital to analog converter. The displaying of a video information with such a structure of AM-OLED is the following. The input signal is forwarded to the digital processing unit that delivers, after internal processing, a timing signal for row selection to the row driver synchronized with the data sent to the column drivers. The data transmitted to the column driver are either parallel or serial. Additionally, the column driver disposes of a reference signaling delivered by a separate reference signaling device. This component delivers a set of reference voltages in case of voltage driven circuitry or a set of reference currents in case of current driven circuitry. The highest reference is used for the white and the lowest for the black level. Then, the column driver applies to the matrix cells the voltage or current amplitude corresponding to the data to be displayed by the cells.
In order to illustrate this concept, an example of a voltage driven circuitry is described below. Such a circuitry will also used in the rest of the present specification for illustrating the invention. The driver taken as example uses 8 reference voltages named V0 to V7 and the video levels are built as shown below:
Video levelGrayscale voltage levelOutput Voltage0V70.00V1V7 + (V6 − V7) × 9/11750.001V2V7 + (V6 − V7) × 32/11750.005V3V7 + (V6 − V7) × 76/11750.011V4V7 + (V6 − V7) × 141/11750.02V5V7 + (V6 − V7) × 224/11750.032V6V7 + (V6 − V7) × 321/11750.045V7V7 + (V6 − V7) × 425/11750.06V8V7 + (V6 − V7) × 529/11750.074V9V7 + (V6 − V7) × 630/11750.089V10V7 + (V6 − V7) × 727/11750.102V11V7 + (V6 − V7) × 820/11750.115V12V7 + (V6 − V7) × 910/11750.128V13V7 + (V6 − V7) × 998/11750.14V14V7 + (V6 − V7) × 1086/11750.153V15V60.165V16V6 + (V5 − V6) × 89/10970.176V17V6 + (V5 − V6) × 173/10970.187V18V6 + (V5 − V6) × 250/10970.196V19V6 + (V5 − V6) × 320/10970.205V20V6 + (V5 − V6) × 386/10970.213V21V6 + (V5 − V6) × 451/10970.221V22V6 + (V5 − V6) × 517/10970.229V. . .. . .. . .250V1 + (V0 − V1) × 2278/30292.901V251V1 + (V0 − V1) × 2411/30292.919V252V1 + (V0 − V1) × 2549/30292.937V253V1 + (V0 − V1) × 2694/30292.956V254V1 + (V0 − V1) × 2851/30292.977V255V03.00V
A more complete table is given in Annex 1. This table illustrates the output voltage for various input video levels. The reference voltages used are for example the following ones:
Reference VnVoltage (Volts)V03V12.6V22.2V31.4V40.6V50.3V60.16V70
Actually, there are three ways for making colored displays                a first possibility illustrated by FIG. 1 is to use a white OLED emitter having on top photopatternable color filters; this type of display is similar to the current LCD displays where the color is also done by using color filters; it has the advantage of using one single OLED material deposition and of having a good color tuning possibility but the efficiency of the whole display is limited by the color filters.        a second possibility illustrated by FIG. 2 is to use blue OLED emitters having on top photopatternable color converters for red and green; such converters are mainly based on materials that absorb a certain spectrum of light and convert it to an other spectrum that is always lower; this type of display has the advantage of using one single OLED material deposition but the efficiency of the whole display is limited by the color converters; furthermore, blue materials are needed since the spectrum of the light can only be reduced by the converters but the blue materials are always the less efficient both in terms of light emission and lifetime.        a third possibility illustrated by FIG. 3 is to use different OLED emitters for the 3 colours red, green and blue. This type of display requires at least 3 material deposition steps but the emitters are more efficient since not filtered.        
The invention is more particularly adapted to the displays of FIG. 3. It can be also used for the other types of display but with fewer advantages.
The use of three different OLED materials (one par color) implies that they all have different behaviors. This means that they all have different threshold voltages and different efficiencies as illustrated by FIG. 4. In the example of FIG. 4, the threshold voltage VBth of the blue material is greater than the threshold voltage VGth of the green material that is itself greater than the threshold voltage VRth of the red material. Moreover, the efficiency of the green material is greater than the efficiencies of the red and blue materials. Consequently, in order to achieve a given color temperature, the gain between these 3 colors must be further adjusted depending on the material color coordinates in the space. For instance, the following materials are used:                Red (x=0.64; y=0.33) with 6 cd/A and VRth=3V        Green (x=0.3; 0.6) with 20 cd/A and VGth=3.3V        Blue (x=0.15; 0.11) with 4 cd/A and VRth=3.5V        
Thus a white color temperature of 6400° K (x=0.313; y=0.328) is achieved by using 100% of the red, 84% of the green and 95% of the blue.
If one driver with only one set of reference signals (voltages or currents) for the 3 colors is used and if the maximum voltage to be applied to the cells is 7 Volts (=Vmax), the voltage range must be from 3V to 7V but only a part of the available dynamic can be used and all corrections must be done digitally. Such a correction will reduce the video dynamic of the whole display. FIG. 5 illustrates the final used video dynamic for the 3 colours. More particularly, the FIG. 5 shows the range used for each diode (colour material) in order to have proper color temperature and black level. Indeed, the minimum voltage Vmin (=V7 in the previous table) to be applied to the diodes must be chosen equal to 3V to enable switching OFF the red diode and the lowest lighting voltage (=V7+(V6−V7)× 9/1175 in the previous table) must be chosen according the blue threshold level to adjust black level. The maximum voltage to be chosen for each diode is adapted to the white color temperature that means 100% red, 84% green and 95% blue. Finally, it can be seen that only a very small part of the green video range is used.
Since the video levels between 3V and 7V are defined with 256 bits, it means that the green component is displayed with only a few digital levels. The red component uses a bit more gray level but this is still not enough to provide a satisfying picture quality. A solution would be to use specific drivers having for all three color outputs a different reference signaling but such drivers are either not available or quite expensive.